You can wait for the bus and change at Orsières, or you can get a taxi all the way. Here's one: http://www.taxi-grand-saint-bernard.ch/7106.html 130 CHF for four people. There are always Haute Routers in the Trient hut that are either looking to share a ride or looking for people to share their ride. Lots of course take the Verbier route and so don't go up to Bourg.